Cellulose acetate film with regulated retardation and thickness

ABSTRACT

A cellulose acetate film is made of a cellulose acetate having an acetic acid content of 59.0 to 61.5%. The film is controlled to have a Re retardation value in the range of 0 to 20 nm, a Rth retardation value in the range of 30 to 70 nm, and a thickness in the range of 10 to 70 nm.

FIELD OF INVENTION

[0001] The present invention relates to a cellulose acetate film havingcontrolled retardation and thickness, and further relates to apolarizing plate and a liquid crystal display employing that film.

BACKGROUND OF INVENTION

[0002] A liquid crystal display comprises a polarizing plate and aliquid crystal cell.

[0003] A TFT (thin film transistor) liquid crystal display of TN(twisted nematic) mode, which is mainly used, further comprises anoptical compensatory film provided between the polarizing plate and theliquid crystal cell. As described in Japanese Patent ProvisionalPublication No. 8(1996)-50206, the optical compensatory film enables thedisplay to give an image of high quality. However, on the other hand,the compensatory film makes the display thick.

[0004] Japanese Patent Provisional Publication No. 1(1989)-68940discloses a liquid crystal display in which an optical compensatorysheet (phase retarder) is provided on one surface of the polarizingplate and an elliptically polarizing plate having a protective film isprovided on the other surface. Because of this structure, the frontcontrast is improved without thickening the display. However, theoptical compensatory sheet is often thermally distorted to change thephase retardation, so that a displayed dark image is framed with leakedlight. Consequently, the display has poor durability.

[0005] In order to improve the durability without thickening thedisplay, Japanese Patent Provisional Publication No. 7(1995)-191217 andEuropean Patent No. 0,911,656A2 disclose that an optical compensatorysheet comprising a transparent support and a thereon-provided opticallyanisotropic layer comprising a discotic compound is directly adopted asa protective film on the polarizing plate. For forming the opticallyanisotropic layer, a coating solution comprising the discotic compoundis applied on the support.

SUMMARY OF INVENTION

[0006] It is found that, in the case where a polarizing plate equippedwith the conventional optical compensatory sheet as the protective filmis attached on a large display panel of 17-inches or more, the framewiselight leakage is not fully prevented. Surprisingly, even if a versatilepolarizing plate is used, the framewise light leakage occurs to impairquality of the displayed image although it has not been hithertoobserved.

[0007] Further, it is also found that, when attached on the largedisplay panel, the polarizing plate is much distorted than when attachedon a display panel of 15 inches or so, and consequently some cracks areoften generated in a cellulose acetate film used as the protective filmof the plate.

[0008] A first object of the present invention is to improve a celluloseacetate film used as a protective film of polarizing plate or as asupport of optical compensatory sheet, and thereby to provide acellulose acetate film excellent in preventing the framewise lightleakage when used in a large liquid crystal display panel.

[0009] A second object of the invention is to provide a polarizing platesuitable for a large liquid crystal display panel.

[0010] A third object of the invention is to provide a large liquidcrystal display panel giving an image of high quality.

[0011] The present inventors have studied and finally found that thelight leakage is caused according to the following two mechanisms.

[0012] One cause of light leakage is change of temperature and humidityunder which a liquid crystal display is left. In the liquid crystaldisplay, a cellulose acetate film used in the polarizing plate or theoptical compensatory sheet is fixed with adhesive on a polarizingmembrane or a liquid crystal cell. When the temperature or humiditychanges, the film expands or shrinks. The expansion or shrinkage islimited since the film is fixed, and accordingly the optical charactersof the film are changed to leak light.

[0013] The other cause is thermal distribution in the cellulose acetatefilm given by heat generated at a backlight in the liquid crystaldisplay. The thermal distribution thermally distorts the film, andaccordingly changes the optical characters to leak light. It has beenalso found that a polymer having hydroxyl group such as celluloseacetate is particularly affected with the change of temperature andhumidity.

[0014] In order to prevent the light leakage, it is desired to make thecellulose acetate film used in the polarizing plate or in the opticalcompensatory sheet less change the optical characters and to reduce thethermal distribution in the film.

[0015] It is found that the change of optical characters depends upon aproduct of photoelastic coefficient, thickness, virtual distortioncaused by change of the environmental conditions, and modulus ofelasticity of the cellulose acetate film. Accordingly, the light leakagecan be remarkably reduced by lowering the photoelastic coefficient, bymaking the film thinner, by reducing the virtual distortion and bymaking the modulus of elasticity smaller. Further, if the thermalconductivity of the film is increased, the thermal distribution in thecellulose acetate film is lowered to reduce the light leakage.

[0016] Particularly when the polarizing plate is attached on a largeliquid crystal display panel, it is expected for the cellulose acetatefilm to crack frequently. The inventors have found that “cracks” in thecellulose acetate film relate to bending strength of the film, and that“cracks” can be reduced if the film is made thinner.

[0017] According to the inventors' study, it is very effective insolving the aforementioned two problems (namely, the light leakage ofliquid crystal display and the cracks of cellulose acetate film) to makethe film thinner.

[0018] The present invention provides a cellulose acetate filmcomprising cellulose acetate having an acetic acid content of 59.0 to61.5%, wherein the Re retardation value defined by the following formula(I) is in the range of 0 to 20 nm, the Rth retardation value defined bythe following formula (II) is in the range of 30 to 70 nm, and thethickness is in the range of 10 to 70 μm:

Re=(nx−ny)×d  (I)

Rth={(nx+ny)/2−nz}×d  (II)

[0019] [in which nx is a refractive index along the slow axis in thefilm plane, ny is a refractive index along the fast axis in the filmplane, nz is a refractive index along the depth of the film, and d isthe thickness of the film in terms of nm].

[0020] The invention also provides a polarizing plate comprising a pairof transparent protective films and a polarizing membrane providedbetween them, at least one of said protective films being a celluloseacetate film comprising cellulose acetate having an acetic acid contentof 59.0 to 61.5%; wherein the cellulose acetate film has the Reretardation value defined by the above formula (I) in the range of 0 to20 nm, the Rth retardation value defined by the above formula (II) inthe range of 30 to 70 nm and a thickness in the range of 10 to 70 μm.

[0021] The invention further provides a liquid crystal displaycomprising a pair of polarizing plates and a liquid crystal cell placedbetween the plates, said polarizing plate comprising a pair oftransparent protective films and a polarizing membrane provided betweenthe films, and at least one of said protective films being a celluloseacetate film comprising cellulose acetate having an acetic acid contentof 59.0 to 61.5%; wherein the cellulose acetate film has the Reretardation value defined by the above formula (I) in the range of 0 to20 nm, the Rth retardation value defined by the above formula (II) inthe range of 30 to 70 nm and a thickness in the range of 10 to 70 μm.

[0022] In the present specification, the term “essentially parallel”means the noticed angle is in the range of the strict angle ±5°. Thisangle allowance is preferably less than ±4°, more preferably less than±30, most preferably less than ±2°. The terms “slow axis”, “fast axis”and “transmission axis” mean the directions giving the maximumrefractive index, the minimum refractive index and the maximumtransmittance, respectively.

[0023] The inventors have succeeded in providing a cellulose acetatefilm very suitable for a protective film of the polarizing plate or fora support of the optical compensatory sheet used in a large liquidcrystal display. If the cellulose acetate film of the invention is usedas a protective film of the polarizing plate or as a support of theoptical compensatory sheet, the large liquid crystal display isprevented from leaking light and at the same time the cellulose acetatefilm is prevented from cracking in producing the liquid crystal display.

[0024] In the present invention, the cellulose acetate film is made thinenough to prevent the light leakage and the cracks. If the film is madethinner, its optical characters are generally changed. However, if onlythe cellulose acetate film is made thin, the resultant polarizing plateor optical compensatory sheet cannot have satisfying optical charactersalthough the light leakage is prevented. In the invention, the celluloseacetate film is made thin while the conventional optical characters aremaintained. As a result, the invention provides an excellent celluloseacetate film which keeps conventional optical characters and whosethickness is thin enough to prevent the light leakage.

[0025] The invention also provides a polarizing plate and an opticalcompensatory sheet comprising the cellulose acetate film of theinvention. If only the polarizing plate or the optical compensatorysheet of the invention is installed instead of a conventional plate orsheet, a liquid crystal display can be easily enlarged without causingtroubles (such as the light leakage). Further, since the celluloseacetate film of the invention is thinner than a conventional film, thepolarizing plate and the optical compensatory sheet can be made thinner.

[0026] The cellulose acetate film of the invention or the polarizingplate whose protective film is an optical compensatory sheet comprisingthe cellulose acetate film of the invention as a support can beadvantageously used in any liquid crystal display having a polarizingplate such as a display of TN (twisted nematic) mode, VA (verticalalignment) mode, IPS (in-plane switching) mode or OCB (opticalcompensate bend) mode.

DETAILED DESCRIPTION OF INVENTION

[0027] (Prevention of Light Leakage)

[0028] In order to prevent the liquid crystal display from leakinglight, the cellulose acetate film of the invention is controlled to havea thickness of 10 to 70 μm. The thickness is preferably in the range of20 to 60 μm, more preferably in the range of 30 to 50 μm.

[0029] The cellulose acetate film preferably has a photoelasticcoefficient of 1.0×10⁻⁶ cm²/kg or less. The cellulose acetate film has amodulus of elasticity preferably in the range of 3,000 Mpa or less, morepreferably in the range of 2,500 Mpa or less. For reducing the virtualdistortion of the cellulose acetate film, the film is preferablystretched biaxially to align the polymer molecules. Otherwise, it isalso preferred to control the film to have a moisture swellingcoefficient of 30×10⁻⁵/%RH or less. The moisture swelling coefficient ismore preferably 15×10⁻⁵ /%RH or less, most preferably 10×10 ⁻⁵/%RH orless. The moisture swelling coefficient indicates how long the sampleexpands when the relative humidity increases under a constanttemperature. The cellulose acetate film preferably has a thermalconductivity of 1 W/(m·K) or more, to ensure an even thermaldistribution.

[0030] The bending strength of the film measured by means of a MITbending tester is preferably 250 times or more, more preferably 300times or more, so as to prevent the film from cracking when attached ona display panel. If the film is made thin, the bending strength isenhanced. (Retardation of film) Retardation values Re and Rth of thefilm are defined by the following formulas (I) and (II):

Re=(nx−ny)×d  (I)

Rth={(nx+ny)/2−nz}×d  (II)

[0031] In the formulas (I) and (II), nx is a refractive index along theslow axis (i.e., along the direction giving the maximum refractiveindex) in the film plane.

[0032] In the formulas (I) and (II), ny is a refractive index along thefast axis (i.e., along the direction giving the minimum refractiveindex) in the film plane.

[0033] In the formula (II), nz is a refractive index along the thicknessdirection of the film.

[0034] In the formulas (I) and (II), d is a thickness of the film interms of nm.

[0035] In the invention, the values Re and Rth are controlled in theranges of 0 to 20 nm and 30 to 70 nm, respectively. The Rth value iscontrolled preferably in the range of 35 to 60 nm, more preferably inthe range of 40 to 50 nm.

[0036] As described above, it is very important to make the celluloseacetate film thin so as to prevent the light leakage of the display andthe cracks of the film.

[0037] However, if only a conventional cellulose acetate film is madethinner, the optical characters (e.g., Re and Rth values) of the filmare changed although the problems such as the light leakage may besolved. As a result, the resultant polarizing plate or opticalcompensatory sheet has poor optical characters.

[0038] For example, if only a cellulose acetate film used as aprotective film of conventional polarizing plate is made thin, theproblems such as the light leakage can be solved but other problems suchas reduction of viewing angle are caused. If such polarizing plate isinstalled in an optical compensatory sheet, the sheet cannot enlarge theviewing angle.

[0039] Accordingly, in the invention, the film is made thin while theretardation of the film is controlled in the above range. The thuscontrolled film of the invention can solve the problems such as thelight leakage with the optical characters maintained. It is describedbelow in detail how the retardation of the film is controlled.

[0040] The cellulose acetate film preferably has a birefringence value(Dn: nx−ny) in the range of 0.00 to 0.002. The film preferably has abirefringence value along the thickness direction {(nx+ny)/2−nz} in therange of 0.001 to 0.04.

[0041] (Cellulose Acetate)

[0042] In the invention, a cellulose acetate having an acetic acidcontent of 59.0 to 61.5% is used.

[0043] The term “acetic acid content” means the amount of combinedacetic acid per one unit weight of cellulose. The acetic acid content isdetermined according to ASTM: D-817-91 (tests of cellulose acetate).

[0044] The film of the invention is made of a cellulose acetate having aviscosity average polymerization degree (DP) of preferably 250 or more,more preferably 290 or more.

[0045] Further, it is also preferred for the cellulose acetate to have anarrow molecular weight distribution of Mw/Mn (Mw and Mn are weight andnumber average molecular weights, respectively) determined by gelpermeation chromatography. The value of Mw/Mn is preferably in the rangeof 1.0 to 1.7, more preferably in the range of 1.3 to 1.65, mostpreferably in the range of 1.4 to 1.6.

[0046] (Retardation Increasing Agent)

[0047] For adjusting the retardation of the cellulose acetate film, anaromatic compound having at least two aromatic rings is used as aretardation increasing agent.

[0048] The aromatic compound is used in an amount of preferably 0.01 to20 weight parts, more preferably 0.05 to 15 weight parts, furtherpreferably 0.1 to 10 weight parts, most preferably 0.5 to 5 weight partsbased on 100 weight parts of cellulose acetate.

[0049] Two or more aromatic compounds may be used in combination. Thearomatic ring may be either an aromatic hydrocarbon ring or an aromaticheterocyclic ring.

[0050] As the aromatic hydrocarbon ring, a six-membered ring (namely, abenzene ring) is particularly preferred.

[0051] The aromatic heterocyclic ring is generally unsaturated. Thearomatic heterocyclic ring is preferably a five-, six- or seven-memberedring, and more preferably a five- or six-membered ring. The aromaticheterocyclic ring generally has double bonds as many as possible. Thehetero-atom in the ring preferably is nitrogen atom, sulfur atom oroxygen atom, and more preferably is nitrogen atom. Examples of thearomatic heterocyclic ring include furan ring, thiophene ring, pyrrolering, oxazole ring, isoxazole ring, thiazole ring, isothiazole ring,imidazole ring, pyrazole ring, furazane ring, triazole ring, pyran ring,pyridine ring, pyridazine ring, pyrimidine ring, pyrazine ring and1,3,5-triazine ring.

[0052] Preferred aromatic rings are benzene ring, furan ring, thiophenering, pyrrole ring, oxazole ring, thiazole ring, imidazole ring,triazole ring, pyridine ring, pyrimidine ring, pyrazine ring and1,3,5-triazine ring. Benzene ring and 1,3,5-triazine ring are morepreferred.

[0053] The aromatic compound preferably contains at least one1,3,5-triazine ring.

[0054] The number of aromatic rings in the aromatic compound ispreferably in the range of 2 to 20, more preferably in the range of 2 to12, further preferably in the range of 2 to 8, and most preferably inthe range of 2 to 6.

[0055] The relation of the two or more aromatic rings is categorizedinto three cases, namely

[0056] (a) the case in which the aromatic rings form a condensed ring,

[0057] (b) the case in which the aromatic rings are connected through asingle bond, and

[0058] (c) the case in which the aromatic rings are connected through alinking group. [In this case, a spiro-bonding is not formed because therings are aromatic.] The relation of the aromatic rings may be any ofthe cases (a) to (c).

[0059] Examples of the condensed ring in the case (a) include indenering, naphthalene ring, azulene ring, fluorene ring, phenanthrene ring,anthracene ring, acenaphthylene ring, naphthacene ring, pyrene ring,indole ring, isoindole ring, benzofuran ring, benzothiophene ring,indolizine ring, benzoxazole ring, benzothiazole ring, benzimidazolering, benztriazole ring, purine ring, indazole ring, chromene ring,quinoline ring, isoquinoline ring, quinolizine ring, quinazoline ring,cinnoline ring, quinoxaline ring, phthalazine ring, pteridine ring,carbazole ring, acridine ring, phenanthridine ring, xanthene ring,phenazine ring, phenothiazine ring, phenoxthine ring, phenoxazine ringand thianthrene ring. Preferred are naphthalene ring, azulene ring,indole ring, benzoxazole ring, benzothiazole ring, benzimidazole ring,benztriazole ring and quinoline ring.

[0060] The single bond in the case (b) is preferably between carbonatoms of the two aromatic rings. Two or more single bonds may connectthe two aromatic rings to form an aliphatic ring or a non-aromatic ringbetween them.

[0061] The linking group in the case (c) is also preferably betweencarbon atoms of the two aromatic rings. The linking group is preferablyan alkylene group, an alkenylene group, an alkynylene group, —CO—, —O—,—NH—, —S— and a combination thereof.

[0062] Examples of the linking group formed by the combination are shownbelow. In each of the following examples, the right and left terminalsmay be reversed.

[0063] c1: —CO—O—

[0064] c2: —CO—NH—

[0065] c3: -alkylene-O—

[0066] c4: —NH—CO—NH—

[0067] c5: —NH—CO—O—

[0068] c6: —O—CO—O—

[0069] c7: —O-alkylene-O—

[0070] c8: —CO-alkenylene—

[0071] c9: —CO-alkenylene-NH—

[0072] c10: —CO-alkenylene-O—

[0073] c11: -alkylene-CO—O-alkylene-O—CO-alkylene-

[0074] c12: —O-alkylene-CO—O-alkylene-O—CO-alkylene-O—

[0075] c13: —O—CO-alkylene-CO—O—

[0076] c14: —NH—CO-alkenylene-c15: —O—CO-alkenylene-

[0077] The aromatic ring and the linking group may have substituentgroups.

[0078] Examples of the substituent group include halogen atoms (F, Cl,Br, I), hydroxyl, carboxyl, cyano, amino, nitro, sulfo, carbamoyl,sulfamoyl, ureido, an alkyl group, an alkenyl group, an alkynyl group,an aliphatic acyl group, an aliphatic acyloxy group, an alkoxy group, analkoxycarbonyl group, an alkoxycarbonylamino group, an alkylthio group,an alkylsulfonyl group, an aliphatic amide group, an aliphaticsulfoneamide group, an aliphatic substituted amine group, an aliphaticsubstituted carbamoyl group, an aliphatic substituted sulfamoyl group,an aliphatic substituted ureido group and a non-aromatic heterocyclicgroup.

[0079] The alkyl group preferably has 1 to 8 carbon atoms. A chain alkylgroup is preferred to a cyclic one, and a straight chain alkyl group isparticularly preferred. The alkyl group may further have a susbstituentgroup (e.g., hydroxyl, carboxyl, an alkoxy group, an alkyl-substitutedamino group). Examples of the (substituted) alkyl group include methyl,ethyl, n-butyl, n-hexyl, 2-hydroxyethyl, 4-carboxybutyl, 2-methoxyethyland 2-diethylaminoethyl.

[0080] The alkenyl group preferably has 2 to 8 carbon atoms. A chainalkenyl group is preferred to a cyclic one, and a straight chain alkenylgroup is particularly preferred. The alkenyl group may further have asusbstituent group. Examples of the alkenyl group include vinyl, allyland 1-hexenyl.

[0081] The alkynyl group preferably has 2 to 8 carbon atoms. A chainalkynyl group is preferred to a cyclic one, and a straight chain alkynylgroup is particularly preferred. The alkynyl group may further have asusbstituent group. Examples of the alkynyl group include ethynyl,1-butynyl and 1-hexynyl.

[0082] The aliphatic acyl group preferably has 1 to 10 carbon atoms.Examples of the aliphatic acyl group include acetyl, propanoyl andbutanoyl.

[0083] The aliphatic acyloxy group preferably has 1 to 10 carbon atoms.Examples of the aliphatic acyloxy group include acetoxy.

[0084] The alkoxy group preferably has 1 to 8 carbon atoms. The alkoxygroup may further have a susbstituent group (e.g., another alkoxygroup). Examples of the (substituted) alkoxy group include methoxy,ethoxy, butoxy and methoxyethoxy.

[0085] The alkoxycarbonyl group preferably has 2 to 10 carbon atoms.Examples of the alkoxycarbonyl group include methoxycarbonyl andethoxycarbonyl.

[0086] The alkoxycarbonylamino group preferably has 2 to 10 carbonatoms. Examples of the alkoxycarbonylamino group includemethoxycarbonylamino and ethoxycarbonylamino.

[0087] The alkylthio group preferably has 1 to 12 carbon atoms. Examplesof the alkylthio group include methylthio, ethylthio and octylthio.

[0088] The alkylsulfonyl group preferably has 1 to 8 carbon atoms.Examples of the alkylsulfonyl group include methanesulfonyl andethanesulfonyl.

[0089] The aliphatic amide group preferably has 1 to 10 carbon atoms.Examples of the aliphatic amide group include acetoamide The aliphaticsulfoneamide group preferably has 1 to 8 carbon atoms. Examples of thealiphatic sulfoneamide group include methanesulfoneamide,butanesulfoneamide and n-octanesulfoneamide.

[0090] The aliphatic substituted amine group preferably has 1 to 10carbon atoms. Examples of the aliphatic substituted amine group includedimethylamino, diethylamino and 2-carboxyethyl amino.

[0091] The aliphatic substituted carbamoyl group preferably has 2 to 10carbon atoms. Examples of the aliphatic substituted carbamoyl groupinclude methylcarbamoyl and diethylcarbamoyl.

[0092] The aliphatic substituted sulfamoyl group preferably has 1 to 8carbon atoms. Examples of the aliphatic substituted sulfamoyl groupinclude methylsulfamoyl and diethylsulfamoyl.

[0093] The aliphatic substituted ureido group preferably has 2 to 10carbon atoms. Examples of the aliphatic substituted ureido group includemethylureido.

[0094] Examples of the non-aromatic heterocyclic group includepiperidino and morpholino.

[0095] The retardation increasing agent has a molecular weight of 300 to800.

[0096] Concrete examples of the retardation increasing agent aredescribed in Japanese Patent Provisional Publication Nos. 2000-111014,2000-275434 and PCT/JP 00/02619.

[0097] (Production of Cellulose Acetate Film)

[0098] The cellulose acetate film is prepared preferably according to asolution-film formation, particularly according to a solvent castingmethod in which an organic solvent is used in the solvent. The solventcasting method comprises the steps of dissolving cellulose acetate inthe organic solvent to prepare a solution (dope) and casting the dope toprepare a film.

[0099] The organic solvent is preferably selected from the groupconsisting of an ether having 3 to 12 carbon atoms, a ketone having 3 to12 carbon atoms, an ester having 3 to 12 carbon atoms and a halogenatedhydrocarbon having 1 to 6 carbon atoms.

[0100] The ether, ketone and ester may have a cyclic structure. Acompound having two or more functional groups of the ether (—O—), ketone(—CO—) and ester (—COO—) can also be used as the organic solvent. Theorganic solvent can have another functional group such as alcoholichydroxyl. In the case where the organic solvent has two or morefunctional group, the number of the carbon atoms is defined as acompound having one functional group, which is optionally selected fromthe aforementioned group of the organic solvents.

[0101] Examples of the ethers having 3 to 12 carbon atoms includediisopropyl ether, dimethoxymethane, dimethoxyethane, 1,4-dioxane,1,3-dioxolane, tetrahydrofuran, anisole and phenetol.

[0102] Examples of the ketones having 3 to 12 carbon atom includeacetone, methyl ethyl ketone, diethyl ketone, diisobutyl ketone,cyclohexanone and methylcyclohexanone.

[0103] Examples of the esters having 3 to 12 carbon atoms include ethylformate, propyl formate, pentyl formate, methyl acetate, ethyl acetateand pentyl acetate.

[0104] Examples of the compounds having two or more kinds of functionalgroups include 2-ethoxyethyl acetate, 2-methoxyethanol and2-butoxyethanol.

[0105] The halogenated hydrocarbon preferably has one or two carbonatoms, and more preferably has one carbon atom. The halogen atom of thehalogenated hydrocarbon preferably is chlorine. The ratio of thesubstitution of hydrogen with halogen is preferably in the range of 25to 75 mole %, more preferably in the range of 30 to 70 mole %, furtherpreferably in the range of 35 to 65 mole %, and most preferably in therange of 40 to 60 mole %. Methylene chloride is a representativehalogenated hydrocarbon.

[0106] Two or more organic solvents can be used in combination.

[0107] A cellulose acetate solution can be prepared according to aconventional method. The conventional method means that the solution isprepared at a temperature of not lower than 0° C. (ordinary or elevatedtemperature). The preparation of the solution can be conducted by usinga process and apparatus for preparation of a dope in a conventionalsolvent casting method. The conventional method preferably uses ahalogenated hydrocarbon (particularly methylene chloride) as an organicsolvent.

[0108] The amount of the polymer is so adjusted that a prepared solutioncontains cellulose ester in an amount of 10 to 40 wt. %. The amount ofcellulose ester more preferably is 10 to 30 wt. %. An optional additive(described below) can be added to the organic (main) solvent.

[0109] The solution can be prepared by stirring cellulose acetate and anorganic solvent at an ordinary temperature (0 to 40° C.). A solution ofa high concentration may be prepared by stirring them at an elevatedtemperature and at a high pressure. In that case, the polymer and theorganic solvent are placed in a closed vessel, and are stirred at anelevated temperature and at a high pressure. The temperature is set tobe higher than the boiling point at atmospheric pressure but lower thanthe boiling point of the solvent at the high pressure. In concrete, theheating temperature is usually not lower than 40° C., preferably in therange of 60 to 200° C., and more preferably in the range of 80 to 110°C.

[0110] The components can be preliminary dispersed coarsely, and thecoarse dispersion can be placed in the vessel. Otherwise, the componentscan also be introduced into the vessel in series. The vessel should beequipped with a stirring device. A pressure in the vessel can be formedby introducing an inert gas such as nitrogen gas into the vessel, or byheating and evaporating the solvent to increase the vapor pressure.Otherwise, the components can be added to the vessel at a high pressureafter the vessel is sealed.

[0111] The vessel is preferably heated outside. For example, the vesselcan be heated with a jacket type heating apparatus. Further, a plateheater can be placed outside the vessel. Furthermore, a heated liquidcan be circulated in a tube outside the vessel.

[0112] The components are stirred preferably with a stirring wing placedin the vessel. The stirring wing has such a length that the end of thewing reaches the wall of the vessel. A scratching tip is preferablyattached to the end of the stirring wing to scratch and renew thesolution remaining on the inside wall of the vessel.

[0113] The vessel can have a meter such as a manometer or a thermometer.The components are dissolved in the solvent in the vessel. The prepareddope is cooled in the vessel, or the dope is cooled after it is takenout of the vessel. The dope can be cooled by a heat exchanger.

[0114] The solution can be also prepared according to a coolingdissolution method. According to the cooling dissolution method, thecellulose acetate can be dissolved even in organic solvents in which itcannot be dissolved according to a conventional method. Further, iforganic solvents in which the cellulose acetate can be dissolvedaccording to a conventional method are used, the cooling dissolutionmethod can prepare a solution more quickly.

[0115] At the first stage of the cooling dissolution method, thecellulose acetate is gradually added to an organic solvent whilestirring at room temperature.

[0116] The amount of cellulose acetate is in the range of 10 to 40 wt. %based on the amount of the mixture, and is preferably in the range of 10to 30 wt. %. An optional additive (described below) may be added to themixture.

[0117] At the next stage, the mixture is cooled to a temperature of −100to −10° C., preferably −80 to −10° C., more preferably −50 to −20° C.,and most preferably −50 to −30° C.

[0118] The mixture can be cooled in a dry ice/methanol bath (−75° C.) orin a cooled diethylene glycol solution (−30 to −20° C.). At the coolingstage, the mixture of the cellulose acetate and the solvent generallysolidify.

[0119] The cooling rate is preferably faster than 4° C. per minute, morepreferably faster than 8° C. per minute, and most preferably faster than12° C. per minute. The cooling rate is preferably as fast as possible.However, a theoretical upper limit of the cooling rate is 10,000° C. persecond, a technical upper limit is 1,000° C. per second, and a practicalupper limit is 100° C. per second. The cooling rate means the change oftemperature at the cooling stage per the time taken to complete thecooling stage. The change of temperature means the difference betweenthe temperature at which the cooling stage is started and thetemperature at which the cooling stage is completed.

[0120] The solidified mixture is warmed to a temperature of 0 to 200°C., preferably 0 to 150° C., more preferably 0 to 120° C., and mostpreferably 0 to 50° C. to dissolve the cellulose acetate in the solvent.The mixture can be warmed by leaving it at room temperature or on a hotbath.

[0121] The warming rate is preferably faster than 4° C. per minute, morepreferably faster than 8° C. per minute, and most preferably faster than12° C. per minute. The warming rate is preferably fast as possible.However, a theoretical upper limit of the warming rate is 10,000° C. persecond, a technical upper limit is 1,000° C. per second, and a practicalupper limit is 100° C. per second. The warming rate means the change oftemperature at the warming stage per the time taken to complete thewarming stage. The change of temperature means the difference betweenthe temperature at which the warming stage is started and thetemperature at which the warming stage is completed.

[0122] Thus a dope is formed as a uniform solution. If the celluloseacetate is not sufficiently dissolved, the cooling and warming steps canbe repeated. The dope is observed with eyes to determine whether thecellulose acetate is sufficiently dissolved or not.

[0123] A sealed vessel is preferably used in the cooling dissolutionmethod to prevent contamination of water, which is caused by dewcondensation at the cooling step. The time for the cooling and warmingstages can be shortened by conducting the cooling step at a highpressure and by conducting the warming step at a low pressure. Apressure vessel is preferably used at a high or low pressure.

[0124] In the case where the cellulose acetate (acetic acid content:60.9%, viscosity average degree of polymerization: 299) is dissolved inmethyl acetate by the cooling dissolution method to form 20 wt. %solution, the solution has a pseudo sol-gel phase transition point atabout 33° C., which is measured by differential scanning calorimetricanalysis (DSC). Under the transition point, the solution forms a uniformgel. Therefore, the solution should be stored at a temperature higherthan the transition point, preferably at a temperature about 10° C.higher than the point. The pseudo sol-gel phase transition point dependson the combined average acetic acid content of cellulose acetate, theviscosity average degree of polymerization, the concentration of thedope and the nature of the solvent.

[0125] From the obtained cellulose acetate solution (dope), a celluloseacetate film is prepared according to a solvent casting method.

[0126] The dope is cast on a drum or a band, and then evaporated to forma polymer film.

[0127] Before casting the dope, the concentration of the dope is soadjusted that the solid content of the dope is in the range of 18 to 35wt. %. The surface of the drum or band is preferably polished to give amirror plane. The casting and drying stages of the solvent cast methodsare described in U.S. Pat. Nos. 2,336,310, 2,367,603, 2,492,078,2,492,977, 2,492,978, 2,607,704, 2,739,069, 2,739,070, British PatentNos. 640,731, 736,892, Japanese Patent Publication Nos. 45(1970)-4554,49(1974)-5614, Japanese Patent Provisional Publication Nos.60(1985)-176834, 60(1985)-203430 and 62(1987)-115035.

[0128] The drum or band preferably has a surface temperature of nothigher than 10° C. when the dope is cast on the support. After castingthe dope, the dope is preferably dried with flowing air for at least 2seconds. The formed film is peeled off the drum or band, and the filmcan be further dried with hot air to remove the solvent remaining in thefilm. The temperature of the air can be gradually elevated from 100 to160° C. The above-mentioned method is described in Japanese PatentPublication No. 5(1993)-17844. According to the method, the time forcasting and peeling steps can be shortened. This method requires thatthe dope be set to gel at the surface temperature of the drum or band.

[0129] A film having two or more layers can be formed by simultaneouslycasting (co-casting) the dopes. Also in this case, the cellulose acetatefilm is preferably prepared according to the solvent cast method. Thedopes are cast on a drum or a band, and then evaporated to form thefilm. Before casting the dopes, the concentration of each dope is soadjusted that the solid content is in the range of 10 to 40 wt. %. Thesurface of the drum or band is preferably polished to give a mirrorplane.

[0130] In the case where two or more cellulose acetate solutions areused, the solutions may be cast from nozzles provided at intervals inthe transferring direction of the support to form a layered film. Thismethod is described in, for example, Japanese Patent ProvisionalPublication Nos. 61(1986)-158414, 1(1989)-122419 and 11(1999)-198285.The solutions may be simultaneously cast from two nozzles to form alayered film. This method is described in, for example, Japanese PatentPublication No. 60(1985)-27562, Japanese Patent Provisional PublicationNos. 61(1986)-94724, 61(1986)-947245, 61(1986)-104813, 61(1986)-158413and 6(1994)-134933.

[0131] The method described in Japanese Patent Provisional PublicationNo. 56(1981)-162617 can be also adopted. In that method, a highlyviscous cellulose acetate solution is enclosed with a low viscous one,and then the thus-combined solutions are simultaneously extruded andcast.

[0132] Further, the method described in, for example, Japanese PatentPublication No. 44(1969)-20235 may be adopted. In the method, a film isbeforehand formed from a solution extruded out of one of two nozzles.After the formed film is peeled and reversely placed on the support,another solution is extruded from the other nozzle to cast onto the film(on the surface having faced to the support) to form a layered film.

[0133] The cellulose acetate solutions may be the same or different fromeach other. Two or more cellulose acetate layers having differentfunctions can be also formed by extruding the corresponding celluloseacetate solutions from the nozzles.

[0134] Further, the cellulose acetate solutions may be cast togetherwith dopes for other functional layers (e.g., adhesive layer, dye layer,antistatic layer, anti-halation layer, ultraviolet-absorbing layer,polarizing layer).

[0135] In the case where a film consisting of a single layer is formedby the solvent casting method, it is necessary to extrude a dope of highconcentration and high viscosity. That dope is generally so unstablethat solid particles are often deposited and that the formed film oftenhas poor evenness. If such viscous dope is extruded simultaneously fromplural nozzles onto the support, a film having excellent evenness can beprepared. Further, since the thick dope is rapidly dried, the film canbe rapidly produced.

[0136] A plasticizer can be added to the cellulose acetate film toimprove the mechanical strength of the film. The plasticizer has anotherfunction of shortening the time for the drying process. Phosphoricesters and carboxylic esters are usually used as the plasticizer.Examples of the phosphoric esters include triphenyl phosphate (TPP) andtricresyl phosphate (TCP). Examples of the carboxylic esters includephthalic esters and citric esters. Examples of the phthalic estersinclude dimethyl phthalate (DMP), diethyl phthalate (DEP), dibutylphthalate (DBP), dioctyl phthalate (DOP), diphenyl phthalate (DPP) anddiethylhexyl phthalate (DEHP). Examples of the citric esters includetriethyl o-acetylcitrate (OACTE) and tributyl o-acetylcitrate (OACTB).Examples of the other carboxylic esters include butyl oleate,methylacetyl ricinoleate, dibutyl sebacate and various trimelliticesters. Phthalic ester plasticizers (DMP, DEP, DBP, DOP, DPP, DEHP) arepreferred. DEP and DPP are particularly preferred.

[0137] The amount of the plasticizer is preferably in the range of 0.1to 25 wt. %, more preferably in the range of 1 to 20 wt. %, and mostpreferably in the range of 3 to 15 wt. % based on the amount of thecellulose acetate.

[0138] Deterioration inhibitors (e.g., antioxidizing agent, peroxidedecomposer, radical inhibitor, metal inactivating agent, oxygenscavenger, amine) can be incorporated into the cellulose acetate film.The deterioration inhibitors are described in Japanese PatentProvisional Publication Nos. 3(1991)-199201, 5(1993)-1907073,5(1993)-194789, 5(1993)-271471 and 6(1994)-107854. The deteriorationinhibitor is preferably added in the range of 0.01 to 1 wt. %, and morepreferably in the range of 0.01 to 0.2 wt. % based on the amount of theprepared solution (dope). If the amount is less than 0.01 wt. %, theeffect of the deterioration inhibitor cannot be expected. If the amountis more than 1 wt. %, the inhibitor often bleeds out on the surface ofthe film. Butyrated hydroxytoluene (BHT) and tribenzylamine (TBA) areparticularly preferred deterioration inhibitors.

[0139] (Highly Thermal Conductive Particles)

[0140] For improving thermal conductivity of the cellulose acetate film,various highly thermal conductive particles can be added to the film.The thermal conductive particles are preferably made of transparentmaterial. Examples of the material include aluminum nitride, siliconnitride, boron nitride, magnesium nitride, silicon carbide, aluminumoxide, silicon oxide, zinc oxide, magnesium oxide, carbon (includingdiamond) and metals.

[0141] The amount of the thermal conductive particles is preferably inthe range of 5 to 100 weight parts based on 100 weight parts of thecellulose acetate. If the amount is less than 5 weight parts, thethermal conductivity of the film is insufficiently improved. If it ismore than 50 weight parts, it is difficult to produce the film and theproduced film is liable to have poor strength.

[0142] The highly thermal conductive particles have a mean particle sizepreferably in the range of 0.05 to 80 μm, more preferably in the rangeof 0.1 to 10 μm. The particles may have either spherical shape orneedle-like shape.

[0143] (Biaxial Stretching)

[0144] For reducing the virtual distortion, the film is preferablystretched. If the film is stretched, the virtual distortion along thestretching direction is reduced. Therefore, the stretching is preferablyperformed so that distortions in all the directions in plane may beevenly inhibited, and hence biaxial stretching is preferred.

[0145] In the biaxial stretching, the film is simultaneously orsuccessively stretched in two directions (namely, simultaneous biaxialstretching or successive biaxial stretching). The successive biaxialstretching is preferred in consideration of continuous production. Theprocess of the successive biaxial stretching comprises the steps ofcasting the dope, peeling the formed film from the band or drum,stretching the film laterally (perpendicularly to the castingdirection), and then stretching the film longitudinally. Thelongitudinal stretching step may be performed prior to the lateralstretching.

[0146] Japanese Patent Provisional Publication Nos. 62(1987)-115035,4(1992)-152125, 4(1992)-284211, 4(1992)-298310 and 11(1999)-48271describe the lateral stretching, which is performed at room temperatureor an elevated temperature. The elevated temperature is preferably belowthe glass transition temperature of the film. The film can be stretchedwhile dried in the film production. Preferably, the film is stretchedwhile the solvent still remains in the film. The longitudinal stretchingcan be performed, for example, by controlling the conveying rollers sothat the speed of winding up the film may be faster than that of peelingthe film. The lateral stretching, on the other hand, can be performed bygradually widening the interval between tenters clipping both sides ofthe conveyed film. Otherwise, after the film is dried, it can bestretched by means of a stretching machine (preferably, the film ismonoaxially stretched by means of a long stretching machine). Theexpanding ratio of stretching (the ratio of length increased bystretching based on the original length) is preferably in the range of 3to 50%, more preferably in the range of 5 to 30%, most preferably in therange of 8 to 20%.

[0147] The steps from casting to drying may be performed under airatmosphere or relatively inert atmosphere (e.g., nitrogen gasatmosphere). For winding up the film, generally used machines can beused. Examples of the winding method include constant tension method,constant torque method, taper tension method and programmed tensioncontrol method by which inner stress is kept constant.

[0148] (Moisture Swelling Coefficient)

[0149] The moisture swelling coefficient is determined by measuring thechange of length of the sample when the relative humidity is changedwith the temperature kept constant.

[0150] The cellulose acetate film has a moisture swelling coefficient ofpreferably 30×10⁻⁵/%RH or less, more preferably 15×10⁻⁵/%RH or less,most preferably 10×10⁻⁵/%RH or less. The less the moisture swellingcoefficient is, the more it is preferred. However, it is normally1.0×110-5 /%RH or more.

[0151] The moisture swelling coefficient is determined in the followingmanner. First, the formed polymer film (phase retarder) is cut toprepare a sample (5 mm width ¥ 20 mm length). One of the ends of thesample is fixed, and the sample is suspended at 25∞C, 20%RH (Ro). At theother (free) end of the sample, a weight of 0.5 g is further suspended.After 10 minutes, the length of the sample (L₀) is measured. Thehumidity is then changed to 80%RH(R₁) while the temperature is kept at25∞C, and the length (L₁) is measured again. From the measured values,the moisture swelling coefficient is calculated according to thefollowing formula. Ten samples are prepared from the same film, and theprocedure is repeated. The calculated values are averaged to determinethe moisture swelling coefficient.

Moisture swelling coefficient [/%RH]={(L ₁ −L ₀)/L ₀}/(R ₁ −R ₀)

[0152] The less free volume the cellulose acetate film has, the less thesize of the film is changed with moisture. The free volume depends uponthe amount of remaining solvent used in film forming process.Accordingly, the less the solvent remains, the less the size changes.

[0153] For reducing the remaining solvent, the film is dried at a hightemperature for long time. However, if too much time is spent fordrying, the productivity is lowered. Accordingly, the amount ofremaining solvent is preferably in the range of 0.01 to 1 wt. %, morepreferably in the range of 0.02 to 0.07 wt. %, most preferably in therange of 0.03 to 0.05 wt. %.

[0154] The amount of remaining solvent is so controlled that thecellulose acetate film may be produced at a low cost with highproductivity.

[0155] As another method for reducing the film-size change withmoisture, a compound having hydrophobic groups is preferably added.Preferred examples of the hydrophobic groups include alkyl groups andphenyl. The compound is preferably selected from the aforementionedplasticizers or deterioration inhibitors. Examples of the compoundinclude triphenyl phosphate (TPP) and tribenzylamine (TBA).

[0156] The amount of the compound having hydrophobic groups ispreferably in the range of 0.01 to 10 wt. %, more preferably in therange of 0.1 to 5 wt. %, most preferably in the range of 1 to 3 wt. %based on the amount of the prepared solution (dope).

[0157] (Surface Treatment of Cellulose Acetate Film)

[0158] The cellulose acetate film is preferably subjected to a surfacetreatment. Examples of the surface treatment include corona dischargetreatment, glow discharge treatment, flame treatment, alkalisaponification treatment and ultraviolet (UV) treatment. In place of thesurface treatment, an undercoating layer (described in Japanese PatentProvisional Publication No. 7(1995)-333433) may be provided.

[0159] The film after subjected to the surface treatment has a surfaceenergy preferably in the range of not less than 55 mN/m, more preferablyin the range of 60 to 75 mN/m.

[0160] For ensuring the evenness of the film, the surface treatment ispreferably performed so that the temperature of the film may be nothigher than Tg (glass transition temperature) of the film (namely, nothigher than 150∞C).

[0161] When used as a protective film of polarizing plate, the celluloseacetate film is preferably subjected to acid or alkali treatment inconsideration of adhesion onto the polarizing membrane. This means thecellulose acetate film is preferably subjected to saponificationtreatment.

[0162] The alkali saponification treatment can be cyclically performedthrough the steps of immersing the film in an alkaline solution,neutralizing with an acidic solution, washing with water and drying.

[0163] Examples of the alkaline solution include aqueous solutions ofpotassium hydroxide and sodium hydroxide. The normality of hydroxyl ionin the alkaline solution is preferably in the range of 0.1 to 3.0 N,more preferably in the range of 0.5 to 2.0 N. The alkaline solution iskept at a temperature preferably in the range of room temperature to 90°C., more preferably in the range of 40 to 70° C.

[0164] The surface energy can be measured by the contact angle method,the wet heating method or the adsorption method (these methods aredescribed in ‘The basic theory and application of wetting [written inJapanese]’, published by Realize Co., Ltd, 1989). The contact anglemethod is preferred. In that method, two solutions having known surfaceenergies are dropped onto the film. The contact angle of each drop ismeasured, and the surface energy of the film is calculated from themeasured contact angles. The contact angle is, by definition, an angle(including the drop) between the film surface and the tangent of thedrop surface at their crossing point.

[0165] (Optically Anisotropic Layer)

[0166] An optically anisotropic layer may be provided on the celluloseacetate film of the invention.

[0167] The optically anisotropic layer may be formed either by directlyapplying a coating solution on the cellulose acetate film or bylaminating, with adhesive, the film of the invention and another film onwhich an optically anisotropic layer is beforehand prepared. Between theoptically anisotropic layer and the cellulose acetate film, anorientation layer is preferably provided.

[0168] The thus-prepared optical compensatory sheet is placed betweenthe polarizing plate and the liquid crystal cell in the liquid crystaldisplay, so as to optically compensate the cell and to prevent the lightleakage. The optical compensatory sheet may be used as a protective filmof the polarizing plate in the liquid crystal display.

[0169] For preparing the optically anisotropic layer, a coating solutioncontaining a liquid crystal compound and other optional components (suchas polymerization initiator) is applied on the orientation layer.

[0170] The liquid crystal compound used in the optically anisotropiclayer may be either rod-like or discotic one. The compound may be apolymer liquid crystal or a monomer liquid crystal of low molecularweight. Further, a polymer in which liquid crystal molecules of lowmolecular weight are crosslinked and hence which no longer behaves asliquid crystal is also usable. A discotic liquid crystal compound ismost preferably used in the invention

[0171] (Rod-Like Liquid Crystal Compound)

[0172] Examples of the rod-like liquid crystal compound includeazomethines, azoxys, cyanobiphenyls, cyanophenyl esters, benzoic esters,phenyl esters of cyclohexanecarboxylates, cyanophenylcyclohexanes,cyano-substituted phenylpyrimidines, alkoxy-substitutedphenylpyrimidines, phenyldioxanes, tolanes, andalkenylcyclohexylbenzonitriles.

[0173] Metal complexes are also included in the rod-like liquid crystalcompounds. Further, a liquid crystal polymer in which the repeating unitcomprises a rod-like liquid crystal moiety is also usable as therod-like liquid crystal compound. In other words, the rod-like liquidcrystal compound may be combined with a (liquid crystal) polymer.

[0174] Descriptions of the rod-like liquid crystal compounds are foundin “Kagaku-Sosetsu, Ekisho no Kageku” (written in Japanese), vol.22(1994), Chapters 4, 7 and 11; and “Ekisho Devise Handbook” (written inJapanese), chapter 3.

[0175] The rod-like liquid crystal compound preferably has abirefringent index of 0.001 to 0.7.

[0176] The molecule of the rod-like liquid crystal compound preferablyhas a polymerizable group (O) to fix the alignment.

[0177] Examples of the polymerizable group (O) are shown below.

[0178] The polymerizable group (O) preferably is an unsaturatedpolymerizable group (Q1 to Q7), an epoxy group (Q8) or an aziridinylgroup (Q9), more preferably is an unsaturated polymerizable group, andmost preferably is an ethylenically unsaturated group (Q1 to Q6).

[0179] The rod-like liquid crystal molecule preferably has an almostsymmetrical structure, and accordingly preferably has a polymerizablegroup at each end.

[0180] Examples of the rod-like liquid crystal molecule are shown below.

[0181] The optically anisotropic layer is prepared by coating theorientation layer with a liquid crystal composition (coating solution)containing the rod-like liquid crystal compound and other optionalcomponents such as polymerization initiator and additives (e.g.,plasticizer, monomer, surface active agent, cellulose acetate,1,3,5-triazine, chiral agent).

[0182] (Discotic Liquid Crystal Compound)

[0183] Examples of the discotic liquid crystal compound include benzenederivatives described in C. Destrade et al., Mol. Cryst. vol. 71, pp.111, (1981); truxene derivatives described in C. Destrade et al., MolCryst. vol. 122, pp. 141. (1985), Physics lett. A, vol. 78, pp. 82,(1990); cyclohexane derivatives described in B. Kohn et al., Angew.Chem. vol. 96, pp. 70, (1984); and macrocyclic compounds ofazacrown-type or phenylacetylene-type described in J. M. Lehn et al., J.Chem. Commun. pp. 1794, (1985), and J. Zhang et al., J. Am. Chem. Soc.vol. 116, pp.2655, (1994).

[0184] The discotic compound has a structure in which the discoticstructure unit is located at the center as a parent core and furtherstraight chain groups such as alkyl, alkoxy and substituted benzoyl areradially substituted. The discotic compound generally has the propertiesof liquid crystal, and hence includes a compound generally calleddiscotic liquid crystal. As the discotic liquid compounds, any compoundcan be used so long as it has negative uniaxial property and orientationproperty. Substance derived from the discotic compound is not always theabove-described compound. For example, the low molecular weight discoticliquid crystal compound having a thermo- or photoreactive group ispolymerized by heat or light to form a polymer that does not behave asliquid crystal. Such polymer can be also used in the invention.Preferred examples of the discotic liquid crystal compound are describedin Japanese Patent Provisional Publication No. 8(1996)-50206. JapanesePatent Provisional Publication No. 8(1996)-27284 describespolymerization of the discotic liquid crystal compound.

[0185] A polymerizable group should be bound to a discotic core of thediscotic compound to cause the polymerization reaction of the compound.However, if the polymerizable group is directly bound to the discoticcore, it is difficult to keep the alignment at the polymerizationreaction. Therefore, a linking group is introduced between the discoticcore and the polymerizable group. Accordingly, the discotic compoundhaving a polymerizable group preferably is a compound represented by thefollowing formula (III):

D(-L-Q)_(n)  (III)

[0186] in which D is a discotic core; L is a divalent linking group; Qis a polymerizable group; and n is an integer of 4 to 12.

[0187] Examples of the discotic cores (D) are shown below. In theexamples, LQ (or QL) means the combination of the divalent linking group(L) and the polymerizable group (O).

[0188] In the formula (III), the divalent linking group (L) preferablyis selected from the group consisting of an alkylene group, analkenylene group, an arylene group, —CO, —NH—, —O—, —S— and combinationsthereof. L more preferably is a divalent linking group comprising atleast two divalent groups selected from the group consisting of analkylene group, an arylene group, —CO—, —NH—, —O— and —S—. L morepreferably is a divalent linking group comprising at least two divalentgroups selected from the group consisting of an alkylene group, anarylene group, —CO— and —O—. The alkylene group preferably has 1 to 12carbon atoms. The alkenylene group preferably has 2 to 12 carbon atoms.The arylene group preferably has 6 to 10 carbon atoms.

[0189] Examples of the divalent linking groups (L) are shown below. Inthe examples, the left side is attached to the discotic core (D), andthe right side is attached to the polymerizable group (O). The AL meansan alkylene group or an alkenylene group. The AR means an arylene group.The alkylene group, the alkenylene group and the arylene group may havea substituent group (e.g., an alkyl group).

[0190] L1: -AL-CO—O-AL-

[0191] L2: -AL-CO—O-AL-O—

[0192] L3: -AL-CO—O-AL-O-AL-

[0193] L4: -AL-CO—O-AL-O—CO—

[0194] L5: —CO-AR-O-AL-

[0195] L6: —CO-AR-O-AL-O—

[0196] L7: —CO-AR-O-AL-O—CO—

[0197] L8: —CO—NH-AL-

[0198] L9: —NH-AL-O—

[0199] L10: —NH-AL-O—CO—

[0200] L11: —O-AL-

[0201] L12: —O-AL-O—

[0202] L13: —O-AL-O—CO—

[0203] L14: —O-AL-O—CO—NH-AL-

[0204] L15: —O-AL-S-AL-

[0205] L16: —O—CO-AR-O-AL-CO—

[0206] L17: —O—CO-AR-O-AL-O—CO—

[0207] L18: —O—CO-AR-O-AL-O-AL-O—CO—

[0208] L19: —O—CO-AR-O-AL-O-AL-O-AL-O—CO—

[0209] L20: —S-AL-

[0210] L21: —S-AL-O—

[0211] L22: —S-AL-O—CO—

[0212] L23: —S-AL-S-AL-

[0213] L24: —S-AR-AL-

[0214] The polymerizable group (O) in the formula (III) is determinedaccording to the polymerization reaction. Examples of the polymerizablegroups (O) are the same as Q1 to Q17 shown above for rod-like liquidcrystal compound.

[0215] The polymerizable group (O) preferably is an unsaturatedpolymerizable group (Q1 to Q7), an epoxy group (Q8) or an aziridinylgroup (Q9), more preferably is an unsaturated polymerizable group, andmost preferably is an ethylenically unsaturated group (Q1 to Q6).

[0216] In the formula (III), n is an integer of 4 to 12, which isdetermined according to the chemical structure of the discotic core (D).The 4 to 12 combinations of L and Q can be different from each other.However, the combinations are preferably identical.

[0217] If the discotic compound is used, the optically anisotropic layershows negative birefringence. In that optically anisotropic layer,discotic structure units of the discotic compound preferably have planesinclined from a plane of the cellulose acetate film at an angle varyingin (along) the direction of depth of the layer.

[0218] The above-described angle (inclined angle) of the plane ofdiscotic structure unit generally increases or decreases with increaseof distance in the direction of depth from the bottom of the opticallyanisotropic layer. The inclined angle preferably increases with increaseof the distance. Further, examples of variation of the inclined angleinclude continuous increase, continuous decrease, intermittent increase,intermittent decrease, variation containing continuous increase anddecrease, and intermittent variation containing increase or decrease.The intermittent variation contains an area where the inclined angledoes not vary in the course of the thickness direction of the layer. Theinclined angle preferably totally increases or decreases in the layer,even if it does not vary in the course. The inclined angle morepreferably increases totally, and it is particularly preferred toincrease continuously.

[0219] The inclined angle of the discotic unit near the support can begenerally controlled by selecting the discotic compound or materials ofthe orientation layer, or by selecting methods for the rubbingtreatment. The inclined angle of the discotic unit near the surface(air) can be generally controlled by selecting the discotic compound orother compounds used together with the discotic compound. Examples ofthe compounds used together with the discotic compound includeplasticizer, surface active agent, polymerizable monomer and polymer.Further, the extent of variation of the inclined angle can be alsocontrolled by the above selection.

[0220] Any compound can be employed as the plasticizer, the surfaceactive agent and the polymerizable monomer, so long as it is compatiblewith the discotic compound and it gives variation of the inclined angleor dose not inhibit the discotic compound molecules from aligning.Preferred is polymerizable monomer (e.g., compounds having a vinyl,vinyloxy, acryloyl or methacryloyl group). Those compounds arepreferably used in the amount of 1 to 50 wt. % (especially 5 to 30 wt.%) based on the amount of the discotic compound.

[0221] Any polymer can be used together with the discotic liquid crystalcompound, so long as it is compatible with the discotic compound and itgives variation of the inclined angle. The polymer is, for example,cellulose ester. Preferred examples of the cellulose ester includecellulose acetate, cellulose acetatepropionate, hydrocypropylcellulose,and cellulose acetatebutylate. In order not to prevent molecules of thediscotic compound from aligning, the amount of the polymer is generallyin the range of 0.1 to 10 wt. %, preferably in the range of 0.1 to 8 wt.%, more preferably in the range of 0.1 to 5 wt. % based on the amount ofthe discotic compound.

[0222] The optically anisotropic layer can be generally prepared by thesteps of coating the orientation layer with a solution of the discoticcompound and other compounds dissolved in a solvent, drying, heating toa temperature for forming a discotic nematic phase, and cooling with theoriented condition (discotic nematic phase) kept. Otherwise, the layercan be prepared by the steps of coating the orientation layer with asolution of the discotic compound and other compounds (e.g.,polymerizable monomer, photopolymerization initiator) dissolved in asolvent, drying, heating to a temperature for forming a discotic nematicphase, polymerizing the heated layer (e.g., by radiation of UV light)and cooling. The transition temperature from discotic nematic phase tosolid phase is preferably in the range of 70 to 300° C., especially 70to 170° C.

[0223] (Fixation of Alignment of Liquid Crystal Compound)

[0224] The aligned discotic liquid crystal molecules can be fixed withthe alignment maintained. The discotic liquid crystal molecules arefixed preferably by a polymerization reaction. The polymerizationreaction can be classified into a thermal reaction with a thermalpolymerization initiator and a photo reaction with a photopolymerization initiator. A photo polymerization reaction is preferred.

[0225] Examples of the photo polymerization initiators includea-carbonyl compounds (described in U.S. Pat. Nos. 2,367,661, 2,367,670),acyloin ethers (described in U.S. Pat. No. 2,448,828), a-hydrocarbonsubstituted acyloin compounds (described in U.S. Pat. No. 2,722,512),polycyclic quinone compounds (described in U.S. Pat. Nos. 2,951,758,3,046,127), combinations of triarylimidazoles and p-aminophenyl ketones(described in U.S. Pat. No. 3,549,367), acridine or phenazine compounds(described in Japanese Patent Provisional Publication No.60(1985)-105667 and U.S. Pat. No. 4,239,850) and oxadiazole compounds(described in U.S. Pat. No. 4,212,970).

[0226] The amount of the photo polymerization initiator is preferably inthe range of 0.01 to 20 wt. %, and more preferably in the range of 0.5to 5 wt. % based on the solid content of the coating solution.

[0227] The light irradiation for the photo polymerization is preferablyconducted with ultraviolet rays.

[0228] The exposure energy is preferably in the range of 20 to 50,000mJ/cm², more preferably in the range of 20 to 5,000 mJ/cm², mostpreferably in the range of 100 to 800 mJ/cm². The light irradiation canbe conducted while the layer is heated to accelerate the photopolymerization reaction. The protective layer may be provided on theoptically anisotropic layer.

[0229] (Orientation Layer)

[0230] The orientation layer has a function of giving an orientationdirection of the liquid crystal molecules. Preferred examples of theorientation layer include a layer of an organic compound (preferablypolymer) subjected to rubbing treatment, an obliquely deposited layer ofan inorganic compound, and a layer having micro grooves. Further, abuilt-up film formed according to Langmuir-Blodgett technique (LBtechnique) from w-tricosanoic acid, dioctadecyldimethylammoniumchlorideor methyl stearate can be used as the orientation layer. In addition, alayer prepared by orienting dielectric materials by application ofelectric field or magnetic field can be employed as the orientationlayer.

[0231] The orientation layer is preferably made of a polymer subjectedto rubbing treatment. As the polymer, polyvinyl alcohol is preferred.Particularly, denatured polyvinyl alcohol having hydrophobic groups ispreferred.

[0232] The orientation layer can be prepared from only one polymer.However, for preparing the orientation layer, a layer made of twocrosslinked polymers is preferably subjected to rubbing treatment. Atleast one of the two polymers is preferably crosslinkable by itself orwith a crosslinking agent.

[0233] The polymers which originally have functional groups or to whichfunctional groups are introduced are reacted with light, heat or pHvariation to form the orientation layer; or otherwise linking groups areintroduced by a reactive crosslinking agent into the polymers so thatthe polymers can be crosslinked to form the orientation layer.

[0234] In a normal process, a coating liquid containing the polymersand, if needed, the crosslinking agent is applied on the celluloseacetate film, and then heated to induce the crosslinking reaction. Thereaction may be caused at any stage from the step of coating the filmwith the coating liquid to the step of producing the resultant sheet.

[0235] In consideration of orientation of the liquid crystal molecules(in the optically anisotropic layer) on the orientation layer, thecrosslinking reaction is preferably caused sufficiently after themolecules are aligned.

[0236] Generally, the coating liquid is applied, heated and dried toform the orientation layer on the cellulose acetate film. In thatprocess, the liquid is preferably heated at such a low temperature thatthe molecules are fully crosslinked at the subsequent heating stage forforming the optically anisotropic layer described below.

[0237] Polymers crosslinkable either by itself or with crosslinkingagents can be used. Some polymers are crosslinkable both by itself andwith crosslinking agents. Examples of the polymers include polymethylmethacrylate, acrylic acid/methacrylic acid copolymer,styrene/maleinimide copolymer, polyvinyl alcohol and denatured polyvinylalcohol, poly(N-methylolacrylamide), styrene/vinyltoluene copolymer,chlorosulfonated polyethylene, nitrocellulose, polyvinyl chloride,chlorinated polyolefin, polyester, polyimide, vinyl acetate/vinylchloride copolymer, ethylene/vinyl acetate copolymer, carboxymethylcellulose, polyethylene, polypropylene, polycarbonate, and organicsubstances such as silan coupling agents.

[0238] Preferred examples are water-soluble polymers such aspoly(N-methylolacrylamide), carboxymethylcellulose, gelatin, polyvinylalcohol and denatured polyvinyl alcohol. Gelatin, polyvinyl alcohol anddenatured polyvinyl alcohol are particularly preferred, and polyvinylalcohol and denatured polyvinyl alcohol are further preferred.

[0239] It is most preferred to use two kinds of polyvinyl alcohols ordenatured polyvinyl alcohols having different polymerization degrees.

[0240] The saponification degree of the polyvinyl alcohol is in therange of 70 to 100%, preferably in the range of 80 to 100%, morepreferably in the range of 85 to 95%. The polymerization degree ispreferably in the range of 100 to 3,000.

[0241] Examples of the denatured polyvinyl alcohol include polyvinylalcohols denatured by copolymerization, by chain transfer and by blockpolymerization. Examples of the denaturing group in the copolymerizationinclude COONa, Si(OX)₃, N(CH₃)₃.Cl, C₉H₁₉COO, SO₃Na and C₁₂H₂₅ (in whichX is hydrogen atom or an alkyl group). Examples of the denaturing groupin the chain transfer include COONa, SH and C₁₂H₂₅. Examples of thedenaturing group in the block polymerization include COOH, CONH₂, COORand C₆H₅ (in which R is an alkyl group).

[0242] Non-denatured or denatured polyvinyl alcohols havingsaponification degrees of 80 to 100% are preferred, and those havingsaponification degrees of 85 to 95% are further preferred.

[0243] The denatured polyvinyl alcohol is preferably a product ofreaction between polyvinyl alcohol and the compound represented by thefollowing formula:

[0244] in which R¹ is an alkyl group, an acryloylalkyl group, amethacryloylalkyl group or an epoxyalkyl group; W is a halogen atom, analkyl group or an alkoxy group; X is an atomic group required to form anactive ester, an acid anhydride or a acid halide; p is 0 or 1; and n isan integer of 0 to 4.

[0245] Hereinafter, such denatured polyvinyl alcohol is referred to as“the particular denatured polyvinyl alcohol”.

[0246] The denatured polyvinyl alcohol is more preferably a product ofreaction between polyvinyl alcohol and the compound represented by thefollowing formula:

[0247] in which X¹ is an atomic group required to form an active ester,an acid anhydride or a acid halide; and m is an integer of 2 to 24.

[0248] The polyvinyl alcohol reacted with the compound represented bythe above formulas are the aforementioned non-denatured polyvinylalcohols or polyvinyl alcohols denatured by copolymerization, by chaintransfer or by block polymerization. Preferred examples of theparticular denatured polyvinyl alcohol are described in Japanese PatentProvisional Publication No. 9(1997)-152509.

[0249] With respect to the polyvinyl alcohol, Japanese PatentProvisional Publication No. 8(1996)-338913 describes the synthesis, themeasurement of visible absorption spectrum and how to determine theamount of introduced denaturing groups.

[0250] Examples of the crosslinking agent include aldehydes, (e.g.,formaldehyde, glyoxal, glutaraldehyde), N-methylol compounds (e.g.,dimethylol urea, methyloldimethylhydantoin), dioxane derivatives (e.g.,2,3-dihydroxydioxane), compounds that works when the carboxylic group isactivated (e.g., carbenium, 2-naphthalenesulfonate,1,1-bispyrrolidino-1-chloropyridinium,1-morpholinocarbonyl-3-(sulfonatoaminomethyl), active vinyl compounds(e.g., 1,3,5-triacryloyl-hexahydro-s-triazine,bis-(vinylsulfone)methane,N,N′-methylenebis-[b-vinylsulfonyl]propionamide), active halogencompounds (e.g., 2,4-dichloro-6-hydroxy-s-triazine), isooxazoles anddialdehyde starch. Two or more crosslinking agents may be used incombination.

[0251] They are used together with preferably water-soluble polymer,more preferably polyvinyl alcohol or denatured polyvinyl alcohol(including the above particular polyvinyl alcohol). Reactive aldehydesare preferred, and glutaraldehyde is particularly preferred inconsideration of productivity.

[0252] The amount of the crosslinking agent is not particularlyrestricted. The more the crosslinking agent is added, the more thedurability against moisture is improved. However, if the amount ofcrosslinking agent is 50 wt. % or more per the amount of the polymer,the resultant orientation layer poorly aligns the molecules.Accordingly, the amount of crosslinking agent is preferably in the rangeof 0.1 to 20 wt. %, more preferably in the range of 0.5 to 15 wt. %based on the amount of the polymer. Even after the crosslinking reactionis completed, the obtained orientation layer contains non-reactedcrosslinking agent a little. The amount of the non-reacted crosslinkingagent remaining in the orientation layer is preferably not more than 1.0wt. %, more preferably not more than 0.5 wt. % based on the amount ofthe orientation layer. If the layer contains the non-reacted agent in anamount of more than 1.0 wt. %, the layer has poor durability. A liquidcrystal display comprising such orientation layer often suffers troublesof reticulation if used for a long time or left under hot and humidcondition.

[0253] The orientation layer can be formed by the steps of coating thecellulose acetate film with a coating liquid containing the abovepolymer and the crosslinking agent, heating to dry and crosslink theapplied polymer, and subjecting the formed layer to rubbing treatment.The crosslinking reaction may be caused at any step after applying thecoating liquid.

[0254] In the case where a water-soluble polymer such as polyvinylalcohol is used, the coating solution is preferably prepared from amixed solvent of water and an organic solvent having defoaming character(e.g., methanol). The ratio of water/methanol is normally in the rangeof 0:100 to 99:1, preferably in the range of 0:100 to 91:9. Because ofdefoaming character of the organic solvent, defects on the orientationlayer are remarkably decreased, and accordingly the opticallyanisotropic layer has an improved surface.

[0255] As the coating method, known methods such as spin-coating,dip-coating, curtain-coating, extrusion-coating, bar-coating and Etype-coating can be adopted. The E type-coating method is particularlypreferred.

[0256] The thickness of the layer is preferably in the range of 0.1 to10 μm. The applied layer can be dried at a temperature of 20 to 110° C.For ensuring sufficient crosslinking, the temperature is preferably inthe range of 60 to 100° C., more preferably in the range of 80 to 100°C. The time for drying is in the range of 1 minute to 36 hours,preferably in the range of 5 minute to 30 minutes. The pH is alsopreferably adjusted at an optimal value according to the usedcrosslinking agent. If glutaraldehyde is used as the crosslinking agent,the pH is preferably in the range of 4.5 to 5.5, more preferably at 5.0.

[0257] The rubbing treatment can be conducted in the manner adoptedwidely for aligning liquid crystal molecules of LCD. The surface of thelayer is rubbed with paper, cloth (gauze, felt, nylon, polyester) orrubber along a certain direction, to give the aligning function.Generally, the layer is rubbed several times with cloth on which fibershaving the same length and thickness are provided.

[0258] (Polarizing Plate)

[0259] The polarizing plate comprises two transparent protective filmsand a polarizing membrane provided between the films. One of theprotective films may be the cellulose acetate film of the invention, andthe other may be a normal cellulose acetate film.

[0260] Examples of the polarizing membrane include an iodine polarizingmembrane, a polyene polarizing membrane and a dichromatic dye polarizingmembrane. The iodine polarizing membrane and the dye polarizing membraneare generally prepared from polyvinyl alcohol films.

[0261] It has been found that the moisture-permeability of theprotective film is important for production of the polarizing plate. Inproducing the polarizing plate, the polarizing membrane and theprotective film are laminated with an aqueous adhesive, and then thesolvent of the adhesive is diffused into the film to dry. The higherpermeability the film has, the more rapidly it is dried. Accordingly,the productivity of the polarizing plate is improved. However, if thepermeability is too high, moisture in air is liable to come into themembrane to impair polarizability if the liquid crystal display is usedunder humid condition.

[0262] In the case where the cellulose acetate film of the invention isused as the protective film of the polarizing plate, the film has amoisture-permeability preferably in the range of 100 to 1,000 (g/m²)/24hours, more preferably in the range of 300 to 700 (g/m²)/24 hours.

[0263] The moisture-permeability depends upon various conditions such asthickness, free volume, and hydrophilicity (hydrophobicity) of thepolymer film (and the polymerizable liquid crystal compound).

[0264] In the film forming process, the thickness of the celluloseacetate film can be controlled by the conditions and procedures such asrip flow, line speed, stretching and/or compressing. According thematerials, the thickness is controlled so that the preferredpermeability can be obtained.

[0265] Also in the film forming process, the free volume of thecellulose acetate film can be controlled by drying conditions such astime and temperature. According to the materials, the free volume iscontrolled so that the preferred permeability can be obtained.

[0266] The hydrophilicity (hydrophobicity) of the cellulose acetate filmcan be controlled with additives. If hydrophilic additives are containedin the free volume, the permeability is increased. If hydrophobicadditives are added, the permeability is decreased.

[0267] The moisture-permeability can be thus controlled, and thereby thepolarizing plate having optical compensatory function can be produced atsmall cost with high productivity.

[0268] (Liquid Crystal Display)

[0269] The aforementioned cellulose acetate film or a polarizing platecomprising the film is advantageously used in a liquid crystal display,particularly in a liquid crystal display of transmission type.

[0270] A liquid crystal display of transmission type comprises a pair ofpolarizing plates and a liquid crystal cell placed between them. Thepolarizing plate comprises a pair of transparent protective films and apolarizing membrane placed between them. The liquid crystal cellcomprises a pair of electrode substrates and liquid crystal providedbetween them.

[0271] The cellulose acetate film of the invention is placed between thecell and one or each of the polarizing plates.

[0272] The polarizing plate comprising the cellulose acetate film of theinvention as a protective film can be used in place of one or each ofthe two polarizing plates in the liquid crystal display.

[0273] The liquid crystal cell works preferably according to TN mode, VAmode. IPS mode or OCB mode.

[0274] In a liquid crystal cell of TN mode, rod-like liquid crystalmolecules are essentially horizontally aligned while voltage is notapplied, and oriented in a twisted alignment with a twisted angle of 60to 120°. The liquid crystal cell of TN mode is widely used in color TFTliquid crystal displays, and hence is described in many publications.

[0275] In a liquid crystal cell of VA mode, rod-like liquid crystalmolecules are essentially vertically aligned while voltage is notapplied.

[0276] The liquid crystal cell of VA mode include some types: (1) aliquid crystal cell of VA mode in a narrow sense (described in JapanesePatent Provisional Publication No. 2(1990)-176625 and Japanese PatentPublication No. 7(1995)-69536), in which rod-like liquid crystalmolecules are essentially vertically aligned while voltage is notapplied, and the molecules are essentially horizontally aligned whilevoltage is applied; (2) a liquid crystal cell of MVA mode (described inSID97, Digest of tech. Papers, 28(1997), 845), in which the VA mode ismodified to be multi-domain type so as to enlarge the viewing angle; (3)a liquid crystal cell of n-ASM mode (described in Abstracts of JapaneseForum of Liquid Crystal (written in Japanese), (1998), pp. 58 to 59), inwhich rod-like liquid crystal molecules are essentially verticallyaligned while voltage is not applied, and the molecules are essentiallyoriented in twisted multi-domain alignment while voltage is applied; (4)a cell of SURVAIVAL mode (presented in LCD International '98); and (5) acell of CPA mode (presented in SID 2001).

[0277] A cell of IPS mode is described in detail in, for example, AsiaDisplay '95, 577(1995), M. Oh-e, M. Ohta, S. Aratani, K. Kondo et. al.).

[0278] The liquid crystal cell of OCB mode is a liquid crystal cell ofbend alignment mode in which rod-like liquid crystal molecules in upperpart and ones in lower part are essentially reversely (symmetrically)aligned. A liquid crystal display having the liquid crystal cell of bendalignment mode is disclosed in U.S. Pat. Nos. 4,583,825 and 5,410,422.Since rod-like liquid crystal molecules in upper part and ones in lowerpart are symmetrically aligned, the liquid crystal cell of bendalignment mode has self-optical compensatory function. Therefore, thismode is referred to as OCB (optically compensatory bend) mode. Theliquid crystal display of bend alignment mode has an advantage ofresponding rapidly.

EXAMPLE 1

[0279] The following components were placed in a mixing tank, heated andstirred to dissolve, to prepare a cellulose acetate solution. Celluloseacetate solution Cellulose acetate (acetic acid content: 60.9%) 100weight parts Triphenyl phosphate (plasticizer) 7.8 weight partsBiphenyldiphenyl phosphate (plasticizer) 3.9 weight parts Methylenechloride (first solvent) 300 weight parts Methanol (second solvent) 54weight parts 1-Butanol (third solvent) 11 weight parts

[0280] In another mixing tank, 16 weight parts of the followingretardation increasing agent, 80 weight parts of methylene chloride and20 weight parts of methanol were placed, heated and stirred, to preparea retardation increasing agent solution.

[0281] The cellulose acetate solution (479 weight parts) and theretardation increasing agent solution (21 weight parts) were mixed andstirred well to prepare a dope. The prepared dope contained theretardation increasing agent in the amount of 3.0 weight parts based on100 weight parts of cellulose acetate.

[0282] Retardation Increasing Agent

[0283] The dope was cast on a band by means of a band-casting machine.After the film temperature on the band reached 40° C., the film waspeeled and dried with hot air at 140° C. Thus, a cellulose acetate film(thickness: 40 μm) in which the solvent remained in the amount of 0.3wt. % was obtained.

[0284] The Re and Rth retardation values of the obtained celluloseacetate film (CAF-01) were measured at 550 nm by means of anellipsometer [M-150, JASCO Corporation]. The results are set forth inTable 1.

[0285] (Bending Test)

[0286] The prepared cellulose acetate film (CAF-01) was cut to obtain asample piece of 120 mm. According to IS08776/2-1988, the sample wassubjected to the bending test by means of a MIT bending tester. As aresult, the sample was 310 times bent to break off.

EXAMPLE 2

[0287] The following components were placed in a mixing tank, heated andstirred to dissolve, to prepare a cellulose acetate solution. Celluloseacetate solution Cellulose acetate (acetic acid content: 60.9%) 100weight parts Triphenyl phosphate (plasticizer) 7.8 weight partsBiphenyldiphenyl phosphate (plasticizer) 3.9 weight parts Methylenechloride (first solvent) 300 weight parts Methanol (second solvent) 54weight parts 1-Butanol (third solvent) 11 weight parts

[0288] In another mixing tank, 16 weight parts of the above retardationincreasing agent, 80 weight parts of methylene chloride and 20 weightparts of methanol were placed, heated and stirred, to prepare aretardation increasing agent solution.

[0289] The cellulose acetate solution (486 weight parts) and theretardation increasing agent solution (14 weight parts) were mixed andstirred well to prepare a dope. The prepared dope contained theretardation increasing agent in the amount of 2.0 weight parts based on100 weight parts of cellulose acetate.

[0290] The dope was cast on a band by means of a band-casting machine.After the film temperature on the band reached 40° C., the film wasdried for 1 minute and then peeled off. While the volatile componentremaining in the film was kept 15%, the film was uniaxially stretched inlateral direction by 10% with tenter. The film was then uniaxiallystretched in longitudinal (transferring) direction by 10% while driedwith hot air at 130° C., and further dried with hot air at 140° C. for10 minutes to prepare a cellulose acetate film (thickness: 40 μm) inwhich the solvent remained in the amount of 0.3 wt. %.

[0291] The Re and Rth retardation values of the obtained celluloseacetate film (CAF-02) were measured at 550 nm by means of anellipsometer [M-150, JASCO Corporation]. The results are set forth inTable 1.

[0292] (Bending Test)

[0293] The prepared cellulose acetate film (CAF-02) was cut to obtain asample piece of 120 mm. According to IS08776/2-1988, the sample wassubjected to the bending test by means of a MIT bending tester. As aresult, the sample was 280 times bent to break off.

COMPARISON EXAMPLE 1

[0294] The cellulose acetate solution was directly used as the dope, toprepare a cellulose acetate film (thickness: 110 μm) in which thesolvent remained in the amount of 3.0 wt. %.

[0295] The Re and Rth retardation values of the obtained celluloseacetate film (CAF-H1) were measured at 550 nm by means of anellipsometer [M-150, JASCO Corporation]. The results are set forth inTable 1.

[0296] (Bending Test)

[0297] The prepared cellulose acetate film (CAF-H1) was cut to obtain asample piece of 120 mm. According to IS08776/2-1988, the sample wassubjected to the bending test by means of a MIT bending tester. As aresult, the sample was 180 times bent to break off. TABLE 1 FilmRetardation increasing agent Re Rth Ex. 1 CAF-01 3.0 weight parts 7 nm40 nm Ex. 2 CAF-02 2.0 weight parts 2 nm 45 nm C. Ex. 1 CAF-H1 — 4 nm 48nm

EXAMPLE 3

[0298] Iodine was adsorbed on a stretched polyvinyl alcohol film toprepare a polarizing membrane. The cellulose acetate film (CAF-01)prepared in Example 1 was then laminated on one surface of thepolarizing membrane with a polyvinyl alcohol adhesive.

[0299] On the other surface of the membrane, a commercially availablecellulose triacetate film [FUJITAC TD80UF, Fuji Photo Film Co., Ltd.]was saponified and laminated with a polyvinyl alcohol adhesive. Thus, apolarizing plate was prepared.

EXAMPLE 4

[0300] Iodine was adsorbed on a stretched polyvinyl alcohol film toprepare a polarizing membrane. The cellulose acetate film (CAF-02)prepared in Example 2 was then laminated on one surface of thepolarizing membrane with a polyvinyl alcohol adhesive.

[0301] On the other surface of the membrane, a commercially availablecellulose triacetate film [FUJITAC TD80UF, Fuji Photo Film Co., Ltd.]was saponified and laminated with a polyvinyl alcohol adhesive. Thus, apolarizing plate was prepared.

COMPARISON EXAMPLE 2

[0302] Iodine was adsorbed on a stretched polyvinyl alcohol film toprepare a polarizing membrane. The cellulose acetate film (CAF-H1)prepared in Comparison Example 1 was then laminated on one surface ofthe polarizing membrane with a polyvinyl alcohol adhesive.

[0303] On the other surface of the membrane, a commercially availablecellulose triacetate film [FUJITAC TD80UF, Fuji Photo Film Co., Ltd.]was saponified and laminated with a polyvinyl alcohol adhesive. Thus, apolarizing plate was prepared.

EXAMPLE 5

[0304] A pair of polarizing plates were removed from a commerciallyavailable liquid crystal display (20-inches liquid crystal TV, SharpCorporation), which had a liquid crystal cell of TN mode. In place ofthe removed members, the polarizing plate prepared in Example 3 waslaminated on each side (each of the backlight side and the observerside) of the cell with an adhesive so that the CAF-01 film (celluloseacetate film prepared in Example 1) might be on the liquid crystal cellside. The polarizing plates were arranged so that the transmission axesmight be perpendicularly crossed, to prepare a liquid crystal display.

EXAMPLE 6

[0305] A pair of polarizing plates were removed from a commerciallyavailable liquid crystal display (20-inches liquid crystal TV, SharpCorporation), which had a liquid crystal cell of TN mode. In place ofthe removed members, the polarizing plate prepared in Example 4 waslaminated on each side (each of the backlight side and the observerside) of the cell with an adhesive so that the CAF-02 film (celluloseacetate film prepared in Example 2) might be on the liquid crystal cellside. The polarizing plates were arranged so that the transmission axesmight be perpendicularly crossed, to prepare a liquid crystal display.

COMPARISON EXAMPLE 3

[0306] A pair of polarizing plates were removed from a commerciallyavailable liquid crystal display (20-inches liquid crystal TV, SharpCorporation), which had a liquid crystal cell of TN mode. In place ofthe removed members, the polarizing plate prepared in Comparison Example2 was laminated on each side (each of the backlight side and theobserver side) of the cell with an adhesive so that the CAF-H1 film(cellulose acetate film prepared in Comparison Example 1) might be onthe liquid crystal cell side. The polarizing plates were arranged sothat the transmission axes might be perpendicularly crossed, to preparea liquid crystal display.

[0307] (Evaluation of Frame-Like Light Leakage)

[0308] Each of the liquid crystal displays prepared in Examples 5, 6 andComparison Example 3 was left at the temperature of 25° C. and therelative humidity of 65% for 5 hours while the backlight was kept on.After that, it was observed with the eyes whether each displayframe-like leaked light or not. The results are set forth in Table 2.TABLE 2 Liquid crystal display Light leakage Example 5 Not observedExample 6 Not observed Comparison Example 3 Light leaked frame-like

1. A cellulose acetate film comprising cellulose acetate having anacetic acid content of 59.0 to 61.5%, wherein the film has a Reretardation value defined by the following formula (I) is in the rangeof 0 to 20 nm, a Rth retardation value defined by the following formula(II) in the range of 30 to 70 nm, and a thickness in the range of 10 to70 μm: Re=(nx−ny)×d  (I) Rth={(nx+ny)/2−nz}×d  (II) in which nx is arefractive index along the slow axis in the film plane; ny is arefractive index along the fast axis in the film plane; nz is arefractive index along the depth of the film; and d is the thickness ofthe film in terms of nm.
 2. The cellulose acetate film as defined inclaim 1, wherein the film contains an aromatic compound having at leasttwo aromatic rings in an amount of 0.01 to 20 weight parts based on 100weight parts of cellulose acetate.
 3. The cellulose acetate film asdefined in claim 2, wherein the aromatic compound has at least one1,3,5-triazine ring.
 4. The cellulose acetate film as defined in claim1, wherein the film has bending strength of 250 times or more, which ismeasured by means of a MIT bending tester.
 5. The cellulose acetate filmas defined in claim 1, wherein the film has been prepared from acellulose acetate solution according to a solvent cast method.
 6. Thecellulose acetate film as defined in claim 5, wherein the film has beenprepared by casting two or more cellulose acetate solutionssimultaneously.
 7. The cellulose acetate film as defined in claim 5,wherein the solvent of the cellulose acetate solution contains an etherhaving 3 to 12 carbon atoms, a ketone having 3 to 12 carbon atoms or anester having 3 to 12 carbon atoms.
 8. The cellulose acetate film asdefined in claim 5, wherein the solvent of the cellulose acetatesolution remains in an amount of 0.01 to 1 wt. % based on the amount ofthe film.
 9. The cellulose acetate film as defined in claim 1, whereinthe film is biaxially stretched.
 10. The cellulose acetate film asdefined in claim 1, wherein the film contains a plasticizer in an amountof 0.1 to 25 wt. % based on the amount of cellulose acetate.
 11. Apolarizing plate comprising a pair of transparent protective films and apolarizing membrane provided between them, at least one of saidprotective films being a cellulose acetate film comprising celluloseacetate having an acetic acid content of 59.0 to 61.5%, wherein thecellulose acetate film has a Re retardation value defined by thefollowing formula (I) in the range of 0 to 20 nm, a Rth retardationvalue defined by the following formula (II) in the range of 30 to 70 nmand a thickness in the range of 10 to 70 μm: Re= ( nx−ny)×d  (I)Rth={(nx+ny)/2−nz}×d  (II) in which nx is a refractive index along theslow axis in the film plane; ny is a refractive index along the fastaxis in the film plane; nz is a refractive index along the depth of thefilm; and d is the thickness of the film in terms of nm.
 12. Thepolarizing plate as defined in claim 11, wherein the cellulose acetatefilm contains an aromatic compound having at least two aromatic rings inan amount of 0.01 to 20 weight parts based on 100 weight parts ofcellulose acetate.
 13. A liquid crystal display comprising a pair ofpolarizing plates and a liquid crystal cell placed between the plates,said polarizing plate comprising a pair of transparent protective filmsand a polarizing membrane provided between the films, and at least oneof said protective films being a cellulose acetate film comprisingcellulose acetate having an acetic acid content of 59.0 to 61.5%;wherein the cellulose acetate film has a Re retardation value defined bythe following formula (I) in the range of 0 to 20 nm, a Rth retardationvalue defined by the following formula (II) in the range of 30 to 70 nmand a thickness in the range of 10 to 70 μm: Re=(nx−ny)×d  (I)Rth={(nx+ny)/2−nz}×d  (II) in which nx is a refractive index along theslow axis in the film plane; ny is a refractive index along the fastaxis in the film plane; nz is a refractive index along the depth of thefilm; and d is the thickness of the film in terms of nm.
 14. The liquidcrystal display as defined in claim 13, wherein the cellulose acetatefilm contains an aromatic compound having at least two aromatic rings inan amount of 0.01 to 20 weight parts based on 100 weight parts ofcellulose acetate.